NEUTRON STUDY OF THE MAGNETIC-STRUCTURE CHANGE IN DYMN6GE6

The hexagonal compound DyMn6Ge6 has been studied by neutron diffractio n at various temperatures between T-N=420 K and 11 K. The magnetic str ucture is incommensurate at all temperatures in the ordered regime wit h q=(0, 0, q(x)). The corresponding wavevector component q(z) decrease s strongly on cooling to about T-t=100 K. It remains almost temperatur e independent (q(z)=0.1631) below T-t at which temperature a second se t of Fourier components with q=0 appears. The phase transition at abou t 100 K is shown to be of second order and marks the transition from a triple fiat spiral structure stable at high temperatures to a triple conical structure stable at low temperatures. The origin of the phase transition is the competition between the temperature-dependent Mn and Dy sublattice anisotropies.